Lighting circuits that use light emitting diodes (LEDs) to produce illumination typically have higher energy efficiency and longer service life than equivalent incandescent bulbs, fluorescent lamps, or other lighting sources.
LEDs, however, conduct current in only one direction, and therefore use direct current (DC) to function. In order to function efficiently when powered by an alternating current (AC) power source, a LED-based lighting circuits includes a rectifier circuit to convert a sinusoidal AC input power signal into a half-wave or a full-wave rectified DC power signal. The rectified sinusoidal signal has a variable value that follows a sinusoidal envelope. Because LEDs (and LED lighting circuits) have a threshold voltage below which the LEDs are powered off and neither conduct current or emit light, a LED (or LED lighting circuit) powered by a rectified sinusoidal signal will in general repeatedly turn on and off depending on whether the instantaneous value of the rectified sinusoidal signal exceeds or not the threshold voltage of the LED.
In order to make efficient use of the input power, LED lighting circuits can be designed such that different numbers of LEDs are powered at different times during each cycle. In general, the lighting circuit includes a voltage sensing circuit, for measuring the instantaneous value of the rectified sinusoidal signal, and a microprocessor for determining which LEDs should be powered based on the measured value of the rectified sinusoidal signal. The microprocessor controls a set of digital switches for selectively activating various combinations of LEDs based on the microprocessor's control. For example, the microprocessor may activate a first set of LEDs at the beginning and end of a cycle, when the instantaneous value of the rectified sinusoidal signal is low, and the microprocessor may activate a series connection of two or more sets of LEDs in the middle of the cycle, when the instantaneous value of the rectified sinusoidal signal is high.
The activation and deactivation of the sets of LEDs by the digital switches, however, causes elevated levels of harmonic distortion in the LED lighting circuit and the power lines providing the AC driving signal. In addition, the driving of non-linear LED loads causes power factor distortion in the LED lighting circuit and the power lines providing the AC driving signal. The harmonic and power factor distortions both contribute to decreases in the total efficiency of the LED lighting, as the distortion causes harmonic currents to travel through the power lines providing the AC driving signal.
A need therefore exists for driving circuitry for LED lighting applications which produces minimal total harmonic distortion.